UV/Image Editor - renders both cameras... depending. Is this normal behaviour?
This happens if you lock a camera to a viewport. 

To lock a camera to a specific viewport, go to the specific 3D viewport's header and deselect the camera lock button on the right side of the layer buttons. Then press N, go to "View - Local Camera" and select a different camera. 

To deactivate the view-specific camera, just enable the camera lock and the Render will show the view from the active camera again. 



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